Category:
Water Quality: East Central Illinois
Groundwater Quality
Arsenic and Other Constituents in the Mahomet Aquifer System
Water quality in the Mahomet aquifer and overlying sand and gravel aquifers (the Mahomet aquifer system) is generally very good, with the most common problems being nuisance contaminants, such as iron and hardness, which routinely and easily are removed with treatment. These aquifers are well protected from surface contamination over most of their extents. The most widespread natural contaminant of concern is arsenic, which is found throughout sand and gravel aquifers in Illinois.
Recent ISWS studies1,2 focused on three areas in central Illinois: Champaign County; Tazewell County; and the central part of the Mahomet Bedrock Valley (primarily Piatt, Macon, and Logan Counties). Arsenic concentrations above the drinking water standard (10 µg/L) were found throughout central Illinois, but primarily in Tazewell County and along the bedrock valley walls in the central part of the Mahomet aquifer. Arsenic concentrations appear to be controlled by aquifer geochemistry, primarily organic carbon and sulfate concentrations. There is substantial variability in arsenic concentrations, and the ISWS is unable to provide drillers and well owners with precise information about locating wells to avoid high arsenic zones. As more wells are drilled in the Mahomet aquifer system, more people may be exposed to high arsenic levels from domestic and unregulated wells.
Although arsenic is the main water quality concern in the Mahomet aquifer system, other water quality parameters are of interest, primarily as they contribute to our understanding of flow in the aquifers. In the central part of the Mahomet Bedrock Valley,
there are areas with high TDS concentrations. These occur along the bedrock valley bottom and walls, and are due to bedrock discharge of water with high levels of chloride and sodium into the aquifer. Researchers at the ISWS have been working with scientists from other institutions studying the geochemistry of this region to help determine how the bedrock discharge influences groundwater flow and quality in the Mahomet. It is possible that if the cone of depression west of Champaign continues to grow, the zone of high TDS water may expand.
Groundwater quality is also useful in determining where recharge is occurring and at what rates. One area of interest is the Allerton area in Piatt County, where the Mahomet aquifer is known to be connected to the overlying Glasford aquifer as well as the Sangamon River. Other areas where the Mahomet and Glasford aquifers are connected have recently been discovered.
References
1. Holm, T.R., W.R. Kelly, S. D. Wilson, and G.S. Roadcap, 2004. Arsenic geochemistry and distribution in the Mahomet Aquifer, Illinois. Illinois Waste Management and Research Center, Champaign, Report RR-107, 102 p.
2. Kelly, W.R., T.R. Holm, S.D. Wilson, and G.S. Roadcap, 2005. Arsenic in glacial aquifers: sources and geochemical controls. Ground Water 43(4):500-510.
Surface Water Quality
Important East-Central Illinois Rivers overlying the Mahomet Aquifer System
Impacts of existing and possible future withdrawals of groundwater from the Mahomet aquifer system on the quality of interconnected surface waters such as the Sangamon, Kaskaskia and Middle Fork watersheds are not well known. Potential decreases in base flow may result, with concomitant deleterious effects on water quality and increased water treatment costs. Droughts could produce similar although more temporary effects.
To help anticipate and forecast these water quality effects, a comprehensive water quality database for the Sangamon, Kaskaskia and Middle Fork Rivers, patterned after the Fox River water quality database, should be assembled and examined to determine how important water quality factors (e.g., dissolved oxygen, nutrients, chlorophyll) have varied with past flow conditions. This would provide some clues regarding future impacts, and it is anticipated that important gaps in our knowledge of how water quality relates to flow in these watersheds would be revealed, helping guide future monitoring and modeling efforts.
Surface Water – Groundwater Interactions
Also See: Water Quality
